Schmidt et al., U.S. Pat. No. 3,697,471, disclose a family of polyesterimide resins made by reacting together at least one polybasic acid or a functional derivative thereof, and at least one polyhydric alcohol or functional derivative thereof, at least one of the reactants having at least one five-membered imide ring between the functional groups of the molecule. It is further disclosed that the reactants can be heated in a commercial cresol mixture, then further diluted in a mixture of naphtha and cresol and used as an enamel for coating copper wire to produce a hard, thermally resistant insulation therefor. Meyer et al., U.S. Pat. No. 3,426,098, describe polyesterimide resins in which all or part of the polyhydric alcohol comprises tris(2-hydroxyethyl) isocyanurate.
Sattler, U.S. Pat. No. 3,555,113, describes blends of polymeric amideimideester wire enamels and conductors coated therewith. In Sattler it is suggested that cold blends of polymeric amide-imide-esters and from 20 to 60% of a terephthalic polyester form block copolymers when deposited on a conductor and cured. Such coatings are stated to have better thermal life than coatings from the polyamideimide ester resins alone.
Applicant herein, Pauze, U.S. Pat. No. 3,865,785, describes polyesteramideimide coating compositions with better heat shock properties than the polyesterimide resins alone.
In all cases where polyesterimide resin is used as a coating, smoothness is a problem, especially if higher coating speeds are attempted. Lack of smoothness and blistering not only do not look well, but electrical properties suffer, as is measured by the number of breaks in the insulation in a given length of wire, e.g., 200 feet. The problems can be overcome to some extent by slowing down the coating speed, but this causes losses in energy and productivity.
It has now been discovered that blending a surprisingly small amount of an ester terminated amideimide resin into a major proportion of polyesterimide resin provides a composition which runs rapidly and smoothly on conventional wire coating equipment. The coated wire, as will be seen, is superior both in appearance and in electrical properties to the best coated wires currently obtainable with polyesterimide alone. The blended composition can be used itself, it can be used in heavy builds alone, and it can be used as an undercoat or as an overcoat in dual- or poly-coated conductors of all conventional types.